This invention relates to electroless copper plating baths and more specifically relates to electroless copper bath using neutral ligands based on nitrogen to metal bonds.
Electroless copper plating is widely practiced in the electronics industry, particularly for plating through holes of printed circuit boards by the superior additive process. The current practice of electroless copper plating involves the use of formaldehyde as a reducing agent. Formaldehyde generally requires the operation of the plating bath at a highly alkaline pH, greater than approximately 11. The high pH requirement limits the application of additive copper plating in the presence of alkali sensitive substrates such as polyimide and positive photoresists and possibly ceramic substrates such as aluminum nitride.
In U.S. Pat. No. 4,818,286, entitled "Electroless Copper Plating Bath", and assigned to the same assignee as that of the present application, there is described a plating bath arrangement obviating the requirement of formaldehyde and operating at lower pH.
In the present invention a novel systems approach is applied to electroless plating. Using the approach, the same metal-ligand system is used in a wide variety of buffer systems to formulate stable bath compositions providing acceptable plating performance under varying operating conditions. Such versatility is not possible using existing electroless processes including copper-formaldehyde as described in the article entitled "Electroless Copper Plating Using Dimethylamine Borane" by F. Pearlstein and R. F. Weightman, Plating, May 1973, pages 474-476.